Re: [PATCH v2] perf cpumap: Reduce cpu size from int to int16_t

[Ian Rogers]

On Fri, Jan 3, 2025 at 2:45 PM David Laight
<david.laight.linux@xxxxxxxxx> wrote:
>
> On Fri, 3 Jan 2025 18:25:32 +0000
> Leo Yan <leo.yan@xxxxxxx> wrote:
>
> > On Fri, Dec 20, 2024 at 10:52:07AM -0800, Ian Rogers wrote:
> > >
> > > Fewer than 32k logical CPUs are currently supported by perf. A cpumap
> > > is indexed by an integer (see perf_cpu_map__cpu) yielding a perf_cpu
> > > that wraps a 4-byte int for the logical CPU - the wrapping is done
> > > deliberately to avoid confusing a logical CPU with an index into a
> > > cpumap. Using a 4-byte int within the perf_cpu is larger than required
> > > so this patch reduces it to the 2-byte int16_t. For a cpumap
> > > containing 16 entries this will reduce the array size from 64 to 32
> > > bytes. For very large servers with lots of logical CPUs the size
> > > savings will be greater.
> > >
> > > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > > ---
> > > v2. Rebase and tweak commit message.
> > > ---
> > >  tools/lib/perf/include/perf/cpumap.h |  3 ++-
> > >  tools/perf/util/cpumap.c             | 13 ++++++++-----
> > >  tools/perf/util/env.c                |  2 +-
> > >  3 files changed, 11 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/tools/lib/perf/include/perf/cpumap.h b/tools/lib/perf/include/perf/cpumap.h
> > > index 188a667babc6..8c1ab0f9194e 100644
> > > --- a/tools/lib/perf/include/perf/cpumap.h
> > > +++ b/tools/lib/perf/include/perf/cpumap.h
> > > @@ -4,10 +4,11 @@
> > >
> > >  #include <perf/core.h>
> > >  #include <stdbool.h>
> > > +#include <stdint.h>
> > >
> > >  /** A wrapper around a CPU to avoid confusion with the perf_cpu_map's map's indices. */
> > >  struct perf_cpu {
> > > -       int cpu;
> > > +       int16_t cpu;
> > >  };
> > >
> > >  struct perf_cache {
> > > diff --git a/tools/perf/util/cpumap.c b/tools/perf/util/cpumap.c
> > > index 27094211edd8..85e224d8631b 100644
> > > --- a/tools/perf/util/cpumap.c
> > > +++ b/tools/perf/util/cpumap.c
> > > @@ -427,7 +427,7 @@ static void set_max_cpu_num(void)
> > >  {
> > >         const char *mnt;
> > >         char path[PATH_MAX];
> > > -       int ret = -1;
> > > +       int max, ret = -1;
> > >
> > >         /* set up default */
> > >         max_cpu_num.cpu = 4096;
> > > @@ -444,10 +444,12 @@ static void set_max_cpu_num(void)
> > >                 goto out;
> > >         }
> > >
> > > -       ret = get_max_num(path, &max_cpu_num.cpu);
> > > +       ret = get_max_num(path, &max);
> > >         if (ret)
> > >                 goto out;
> > >
> > > +       max_cpu_num.cpu = max;
> >
> > I am concerned for the data conversion from int type to int16_t type.
> >
> > The GCC option "-Wconversion" is not enabled in perf Makefile, unsafe
> > data conversion is allowed.  A better way is to update argument type for
> > get_max_num() for reading CPU number with int16_t type.
>
> Or just avoid passing &int_var by using the return value instead.
> It'll generate smaller, faster code.

Thanks Leo and David. Changing the argument to get_max_num isn't
possible as the function is used by set_max_node_num and the %d
modifier would need to be different for each, etc. Changing the return
type would have implications as the function uses the int encoding
either 0 as success or a negative value encoding -errno, so there's
the potential for collision between read values from the path and
error numbers. Performance isn't hugely critical here too. I'll send
out a v3 to add a bound check to the int to int16_t assignment, which
hopefully addresses Leo's Wconversion worry.

Thanks,
Ian